This proposal is based on the premise that oxygen free radicals are involved in mitochondrial aging and in turn, aging of the whole organism. Superoxide radicals generated in the mitochondria can lead to damage of macromolecules and result in defective mitochondria. The downward cascade of this process ultimately leads to the state of senescence and the demise of the organism. We hypothesize that factors that can protect the mitochondria from free radical damage have the potential to maintain energy production and tissue function and ultimately to delay the onset of senescence and prolong the lifespan of the organism. Knockout (KO) mice lacking the mitochondrial superoxide metabolizing enzyme, Mn superoxide dismutase (MnSOD), represent an animal model with increased mitochondrial superoxide radicals, accelerated tissue damage, and early demise. We observed a remarkable difference in the mean survival time and the phenotype of the KO mice on different genetic backgrounds. The mean and maximum lifespan difference between the short-lived and the long-lived population is 7 and 5 fold respectively. In addition to the lifespan difference, the long-lived KO mice have a lower level of tissue damage than the short-lived animals. The data indicate that genetic components that cosegregate with the long-lived population have the ability to decelerate tissue damage and consequently, prolong the lifespan. Therefore, identification of these genetic modifiers and understanding their functions protecting mitochondria from superoxide damages may lead to the isolation of genes that can extend lifespan in animal models for human aging. To achieve these goals, the following specific aims are proposed. Aim I - Fine mapping of the major genetic modifier leading to prolonged lifespan in MnSOD mutant mice. Aim II - In vivo and in vitro comparison of lifespan and age- related changes between Sod2-/+ and +/+ animals with and without the genetic modifier. Aim III - Functional studies of the genetic modifier by phenotype analyses of Sod2-/- mice. Aim IV - Identification of the major modifier gene leading to prolonged lifespan in MnSOD mutant mice.